114 



anterior adductor cicatrices distinct, pit-like and deep ; posterior adductor cicatrices shal- 

 low, confluent, that of the retractor pedis muscle impressed at tip of the laterals and 

 below; pallial cicatrix evident, regularly impressed and linear; dorsal cicatrices sev- 

 eral, crowded, in the deep cavity of the umbones or on the iiiarorin of the plate 

 joining the hinge teeth; cavity oi the umbones rather deep; nacre purple, with 

 anterior margin usually white, whole posterior region beautifully iridescent. 



Some interesting features connected with the comparative dimensions of the 



se.ves may be shown from this table of measurements. If the two longest males 



.37.56 37.10 



be selected the ratio of length tn height is = 1.(50 + and 



23.44 



22.32 



.06 



1.66. In 

 37.10 



2.00 and 



17.24 



these same shells the ratio of length to width is as follows: 



18.50 



= 2.15. 



A comparison of the same dimensions for the two longest females develops 



34.40 30.28 

 the following ratios: = 1.55 and =■ 1.50. Comparing the lengths 



22.10 

 with the widths the ratio established is 



20.10 

 34.40 



19.51 



30.28 



1.76 - and = 1.83. The 



16.50 



ratios show that the females are much wider than the males, a relation probably 



due to"the requirements of the ctenidi'i of the female shells when functioning as 



gestatory sacs. So marked, even to casual observation, are these relations that it 



is an easy matter to select the sexes in any considerable number of shells. 



The habits of Unio glans are somewhat different from those ef Unio parvus. 



It more commonly aflfects gravelly beds, in shallow running water. The writer 



has taken the corvuncnlus form in great abundance in the typical locality, whence 



it was traced into nearly all the streams of north (Georgia and Alabama, in the 



Gulf drainage. The ci/lindrellus form is verv abundant in the smaller streams of 



